She said: "I take people through a process of special shampooing to help remove the excess PPD and other chemicals from the hair.

"Dye absorption is similar with fabric as it is to hair, so I take people through a way to remove the chemicals with a pre-fade."

Attempts were made to obtain comment from Schwarzkopf.

In a previous response, a spokesperson said: "The safety of our products is a top priority for Henkel. All of our products, including our hair color products, are designed and manufactured to meet the highest quality standards and applicable legislation, as well as health and safety requirements.

"We strongly advise all consumers to follow the safety instructions on our hair color products, including performing a skin allergy test as recommended on the product insert."